Integrated microelectromechanical system devices and methods for making the same

ABSTRACT

Integrated Microelectromechanical System (“MEMS”) devices and methods for making the same. The integrated MEMS device comprises a substrate ( 200 ) with first electronic circuitry ( 206 ) formed thereon, as well as a MEMS filter device ( 100 ). The MEMS filter device has a transition portion ( 118 ) configured to (a) electrically connect the MEMS filter device to second electronic circuitry and (b) suspend the MEMS filter device over the substrate such that a gas gap exists between the substrate and the MEMS filter device. The transition portion comprises a three dimensional hollow ground structure ( 120 ) in which an elongate center conductor ( 122 ) is suspended. The RF MEMS filter device also comprises at least two adjacent electronic elements ( 102/110 ) which are electrically isolated from each other via a ground structure of the transition portion, and placed in close proximity to each other.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of and claims priority topending non-provisional U.S. patent application Ser. No. 13/970,120filed on Aug. 19, 2013, which is hereby incorporated by reference in itsentirety.

BACKGROUND OF THE INVENTION

Statement of the Technical Field

The inventive arrangements relate to Microelectromechanical System(“MEMS”) and methods for forming the same, and more specifically totransducers with Integrated Circuits (“ICs”).

Description of the Related Art

MEMS is a technology of very small devices typically between 2micrometers to 2 millimeters in size. The MEMS devices can include oneor more components between 1 to 100 micrometers in size. ConventionalMEMS devices are fabricated using molding techniques, platingtechniques, wet etching techniques, dry etching techniques, and/orElectro Discharge Machining (“EDM”) techniques. Various materials can beused to create the MEMS devices. Such materials include silicon,polymers, metals and ceramics.

Radio Frequency filters typically occupy a relatively large amount ofreal estate in an RF system (i.e., >25%). As such, it has been desirableto miniaturize RF filters via MEMS technology, as well as integrate theelectronics with the MEMS RF filters as performance can be achieved. Ingeneral, there are three techniques for integrating MEMS RF filters withICs. The three techniques include a pre-processing technique, apost-process technique, and a merged processing technique. Thepre-processing technique involves: first fabricating a MEMS device(e.g., MEMS RF filter) on a substrate; isolating the MEMS device using adielectric layer; and thereafter fabricating the IC. The post-processingtechnique involves: fabricating the IC on the substrate; isolating theIC using a dielectric layer; and thereafter fabricating the MEMS device.The merged processing technique involves simultaneously fabricating theMEMS device and filter circuitry on a substrate in an interleavingfashion. Typically, the MEMS device is fabricated using a polysiliconmaterial. The ICs are fabricated using thin-film dielectrics and metals(e.g., gold, nickel, aluminum, copper, chromium, titanium, tungsten,platinum and/or silver).

Despite the advantages of integrating RF filters with ICs, theconventional processes for achieving such integration suffer fromcertain drawbacks. For example, the disposition process of a MEMS devicecomprising the polysilicon material requires high temperatures (e.g.,excess of 1000° C.). The materials that are used to build the ICs havemelting points that are much lower (e.g., 300-400° C.) than thetemperatures required to fabricate the MEMS devices. Consequently, theICs may possibly be damaged during the post-processing technique or themerged processing technique as a result. Also, commercially availableconventional integrated RF filters exhibit at best an insertion loss of9 dB, which is considered those skilled in the art as undesirably high.The high insertion loss is primarily due to the use of dielectric films(e.g., Silicon Germanium (“SiGe”)) that are inherently lossy at higherfrequencies as a result of doping.

SUMMARY OF THE INVENTION

The present invention concerns systems and methods for providing anintegrated MEMS device. The MEMS device comprises a substrate, atransition portion, a MEMS filter device, and a gas gap (e.g., an airgap or other dielectric gas gap). The transition portion is coupled toand at least partially extends transversely away from a major surface ofthe substrate. The MEMS filter device is (a) suspended above or over themajor surface of the substrate exclusively by the transition portion,and (b) electrically connected to first electronic circuitry externalthereto by the transition portion. The gas gap exists between the majorsurface of the substrate and the MEMS filter device. Second electroniccircuitry can be formed on the major surface of the substrate using acombination of thin-film dielectrics and metals so as to reside on thesubstrate and isolated from the MEMS filter device. In this scenario,the gas gap exists between the second electronic circuitry and the MEMSfilter device. Notably, an isolation between the MEMS filter device andthe second electronic circuitry can be greater than forty decibels incertain scenarios.

Notably, the MEMS device can be made using a technique which allows theMEMS filter device to be fabricated without the use of high temperaturerequired to manufacture polysilicon based MEMS devices. The fabricationtechnique allows the transition portion to be fabricated with a threedimensional hollow ground structure in which an elongated centerconductor is suspended. The elongated center conductor can be suspendedvia a dielectric strap connected between two opposing sidewalls of thetransition portion. In other instances, the center conductor may besuspended by anchoring to the substrate or some other dielectric wall.Also, the elongated center conductor is separated from the threedimensional hollow ground structure via an air gap on all sides thereof.

The RF MEMS filter device comprises at least two adjacent electronicelements which are electrically isolated from each other via a groundstructure. For example, an electronic element can be electricallyisolated from another element via two spaced apart sidewalls of theground structure. Alternatively, adjacent electronic elements may beelectrically isolated from one another on one side via a common sidewallof the ground structure. In this scenario, the common sidewall has athickness that is at least the thickness of one of the adjoiningsidewalls of the ground structure.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will be described with reference to the following drawingfigures, in which like numerals represent like items throughout thefigures, and in which:

FIG. 1 is a top perspective view of an exemplary MEMS RF filter that isuseful for understanding the present invention.

FIG. 2 is a top perspective view of an exemplary MEMS RF filterfabricated on a substrate that is useful for understanding the presentinvention.

FIG. 3 is a schematic illustration of an exemplary architecture for aMEMS shunt varactor that is useful for understanding the presentinvention.

FIG. 4 is a top-down view of an exemplary MEMS shunt varactor that isuseful for understanding the present invention.

FIGS. 5A-5Z show partial cross-sections of a MEMS device during varioussteps of a fabrication process in accordance with embodiments of thepresent invention.

DETAILED DESCRIPTION

The invention is described with reference to the attached figures. Thefigures are not drawn to scale and they are provided merely toillustrate the instant invention. Several aspects of the invention aredescribed below with reference to example applications for illustration.It should be understood that numerous specific details, relationships,and methods are set forth to provide a full understanding of theinvention. One having ordinary skill in the relevant art, however, willreadily recognize that the invention can be practiced without one ormore of the specific details or with other methods. In other instances,well-known structures or operation are not shown in detail to avoidobscuring the invention. The invention is not limited by the illustratedordering of acts or events, as some acts may occur in different ordersand/or concurrently with other acts or events. Furthermore, not allillustrated acts or events are required to implement a methodology inaccordance with the invention.

The present invention generally concerns MEMS devices which areintegrated with ICs. The MEMS devices can be used in a variety ofapplications. Such applications include, but are not limited to,multi-band communication system applications, radar applications,wide-band tracking receiver applications, broadcast radio applications,television applications, and/or wireless communication deviceapplications (e.g., cellphone applications). The MEMS devices include,but are not limited to, RF filters configured to combine and/or separatemultiple frequency bands, as well as tunable phase shifters. A schematicillustration of an exemplary RF filter 100 is provided in FIG. 1. Insome scenarios, the RF filter 100 comprises a 3-pole tunable bandpassfilter designed to select a desired band of frequencies for a particularfrequency range (e.g., the 1060-1370 MHz range). Embodiments of thepresent invention are not limited to 3-pole tunable bandpass filterarchitectures. The RF filter 100 can include any type of filterarchitecture suitable for a particular application or have as few or asmany poles as necessary for bandwidth. Notably, the tunable feature ofthe RF filter 100 offers significant size reduction over switch-type RFbandpass filter banks.

As shown in FIG. 1, the RF filter 100 is implemented using three shuntvaractors 102, 104, 106, three shunt inductors 108, 110, 112, two seriesinductors 114, 116, and a transition portion 118. Each of the listedcomponents 102-116 can be fabricated using at least one conductivematerial, such as metal (e.g., gold, nickel, aluminum, copper, chromium,titanium, tungsten, platinum, and/or silver). Inductors are well knownin the art, and therefore will not be described in detail herein.However, an exemplary shunt varactor will be described below in relationto FIG. 3. Notably, the RF filter 100 exhibits a 1.9 dB insertion lossacross a 300 MHz bandwidth. This is a significant insertion lossimprovement over conventional RF filter designs, such as those describedabove in the background section of the document.

The transition portion 118 is configured to electrically connect the RFfilter 100 to external circuitry. Accordingly, the transition portion118 comprises a ground structure 120 and a center conductor 122. Thecenter conductor 122 is electrically connected to the shunt varactors102, 104, 106 at points 190, 192, 194, inductors 108, 110, 112 at ends196, 198, 199, and inductors 114, 116 at their outer ends and centerends. Conductive structures 154 are provided to facilitate theelectrical connection of the center conductor 122 to the center ends ofthe inductors 114, 116. Similarly, the ground structure 120 provideselectrical coupling to the shunt varactors 102, 104, 106 via groundingportions 150 and inductors 108, 110, 112 via grounding portions 152.

Operation of inductors are well known in the art, and therefore will notbe described herein. Operation of the shunt varactors 102, 104, 106 willbe described below in relation to FIGS. 3-4. Still, it should beunderstood that each shunt varactor 102, 104, 106 comprisesinterdigitated drive comb structures and a truss comb structure. Avoltage (e.g., 90 Volts) is applied to the drive comb structures via thecenter conductor 122 such that a gap between each drive comb structureand the truss comb structure is varied. For example, in some scenarios,the gap between respective comb structures is varied between 20 micronsdown to 5 microns. Notably, the drive comb structures 170, 172 of eachshunt varactor 102, 104, 106 are electrically connected to each othervia a respective structure 180.

As shown in FIG. 1, the ground structure 120 comprises a plurality ofstraight portions defined by a three dimensional hollow structure with agenerally rectangular cross-sectional profile. The center conductor 122is disposed within the three dimensional hollow structure. In somescenarios, the center conductor 122 is suspended therein so as to extendalong a center axis of each straight portion of the ground structure120. Accordingly, the center conductor 122 is encompassed by the groundstructure 120 along at least a portion of its length and separated fromthe ground structure 120 via an air gap 124 on all sides thereof. Thecenter conductor 122 and ground structure 120 are fabricated using atleast one conductive material, such as metal (e.g., gold, nickel,aluminum, copper, chromium, titanium, tungsten, platinum, and/orsilver).

In some scenarios, the center conductor 122 is suspended within theground structure 120 via one or more dielectric straps (not shown inFIG. 1) connected between opposing sidewalls of the ground structure120. For example, a dielectric strap can be connected between sidewalls160, 162 of the ground structure 120 so as to mechanically supportand/or suspend at least a portion of the center conductor 122 disposedwithin section 164 of the ground structure 120. In some scenarios, aplurality of dielectric straps is disposed along the entire length ofeach straight portion of the ground structure 120.

The ground structure 120 also comprises isolation portions 101, 103,105, 107, 109, 111, 113, 115 each defined by a plurality of sidewalls(e.g., two, three or four sidewalls). Each isolation portion 101, 103,105, 107, 109, 111, 113, 115 at least partially surrounds a respectivecomponent 102, 104, 106, 108, 110, 112, 114, 116 so as to electricallyisolate the same from other adjacent components. For example, as shownin FIG. 1, the ground structure 120 surrounds four sidewalls of eachinductor 108-116. The ground structure 120 also surrounds threesidewalls of each shunt varactor 102-106. Embodiments of the presentinvention are not limited in this regard. Alternatively, the groundstructure 120 can surround one or more sidewalls of one or moreinductors 108-116 and/or shunt varactors 102-106.

In some scenarios, a space 151 is provided between adjacent sidewalls130/132, 134/136, 138/140, 142/144 of the ground structure 120. Notably,the space 151 has dimensions selected for ensuring that adjacentelectronic components are placed in close proximity to each other. Forexample, in some scenarios, the adjacent sidewalls 130/132, 134/136,138/140, 142/144 are spaced 0.1-1.0 mm from each other. In otherscenarios, no space 151 is provided between adjacent sidewalls 130/132,134/136, 138/140, 142/144. Alternatively a single sidewall of the groundstructure 120 (or a “common sidewall”) is used to separate two adjacentcomponents 102/110, 102/114, 104/112, 104/114, 104/116, 106/116,108/104, 108/114, 110/106, 110/114, 110/116, 112/116. In this case, thesingle sidewall has a thickness that is the same as or greater than thatof one adjoining sidewall of the ground structure 120. The otheradjoining sidewalls include, but are not limited to, a sidewall of theground structure to which the common sidewall is adjacent and directlyconnected.

In some scenarios, the RF filter 100 has an overall size of 3.6 mm by4.8 mm. Accordingly, each shunt varactor 102, 104, 106 has a size of 1.1mm by 1.4 mm. Each shunt inductor 108, 110, 112 has a size of 1.1 mm by1.1 mm. Embodiments of the present invention are not limited to theparticularities of such scenarios. However, it should be reiterated thatsuch an RF filer architecture exhibits a 1.9 dB insertion loss across a300 MHz bandwidth. This is a significant insertion loss improvement overconventional RF filter designs, such as those described above in thebackground section of the document.

Notably, the RF filter 100 can be fabricated using a process whichallows the RF filter 100 to be fabricated without the use of the hightemperature required to fabricate conventional polysilicon based MEMSdevices. In some scenarios, the metal material used to fabricate the RFfilter 100 and the metal material used to fabricate an IC have meltingpoints that are the same (e.g., ≤100° C.) or that have no more than a100° C. difference. Embodiments of the present invention are not limitedto the melting point particularities of these scenarios. An exemplaryfabrication process will be described below in relation to FIGS. 5A-5Z.

The fabrication process also allows the RF filter 100 to be fabricatedso as to be suspended above a substrate 200, as shown in FIG. 2.Substrate 200 can include, but is not limited to, a flat semiconductorwafer. In this regard, a gas gap 202 is provided between the RF filter100 and the substrate 200. The gas can include, but is not limited to,air or a dielectric gas. Also, the transition portion 118 has a dualpurpose. Specifically, the transition portion 118 has a dual purpose of(1) electrically connecting the RF filter 100 to external circuitry and(2) mechanically supporting the RF filter 100 structure as its suspendedposition above the substrate 200. The support is achieved by including asupport structure 204 at each end of the transition portion 118. In somescenarios, the support structure 204 has a generally S-like shape with afirst portion connected to the substrate, a second portion extending outand away from the first portion, and a third portion extending alongwith the first portion, as shown in FIG. 2. In other scenarios, thesupport structure 204 simply comprises a vertical straight post-likeshape. A person skilled in the art would understand that otherarchitectures for the support structure 204 can be used herein tosuspend the RF filter 100 above a substrate. Notably, the RF filter 100is suspended over the substrate 200 exclusively by the transitionportion. This arrangement provides certain advantages in the manufactureof the integrated RF filter 100, as well as an improvement in thequality of the RF filter 100 by avoiding use of additional dielectricmaterials (other than air) as additional mechanical supports.

By suspending the RF filter 100 above the substrate 200, valuable spaceon the surface of the substrate 200 is made available for othercircuitry 206, thereby providing a more compact MEMS device as comparedto conventional MEMS devices including RF filters. Notably, there isrelatively minimal coupling (cross talk) of a signal traveling throughthe filter onto circuitry 206 formed below on the substrate 200. Forexample, in some scenarios, the isolation is greater than 40 dB across 6GHz.

Referring now to FIG. 3, there is provided a schematic illustration ofan exemplary architecture for a drive portion 300 of a shunt varactor.Each of the shunt varactors 102, 104, 106 of FIG. 1 can have a driveportion that is the same as or similar to that of FIG. 3. Drive portion300 includes a drive comb structure 302 having a fixed position andextending along a longitudinal axis 304. Drive portion 300 also includesa truss comb structure 306 that extends substantially parallel to axis304 and that can elastically move in the X direction along a motion axis320 substantially parallel to axis 304 of the drive comb structure 302.For example, as shown in FIG. 3, truss comb structure 306 can include orbe attached to at least one restorative or resilient component 112connected to a fixed end. The resilient component 112 restores aposition of truss comb structure 306 when no external forces are beingapplied. The drive comb structure 302 can have one or more drive fingers308 extending therefrom towards truss comb structure 306. The truss combstructure 306 can similarly include one or more truss fingers 310extending therefrom towards the drive comb structure 302.

As shown in FIG. 3, the drive comb structure 302 and the truss combstructure 306 can be positioned to be interdigitating. The term“interdigitating”, as used herein with respect to comb structures,refers to arranging comb structures such that the fingers extending fromsuch comb structures at least partially overlap and are substantiallyparallel.

In the exemplary architecture of FIG. 3, fingers 308 and 310 can eachhave a width and a height of a and b, respectively, and an overlaplength of l. Although comb structures with multiple sets of fingers canbe configured to have the same dimensional relationships (width, height,and overlap), the present invention is not limited in this regard anddimensional relationships can vary, even within a single shunt varactor.Furthermore, the portion shown in FIG. 3 and the dimensionalrelationship shown in FIG. 3 are only the electrically conductiveportions of drive portion 300. As one of ordinary skill in the art willrecognize, comb structures can further include structural portionscomprising non-conductive or semi-conductive materials extending in theZ direction to provide structural support for the conductive portionsshown in FIG. 3.

The drive portion 300 shown in FIG. 3 operates on the principle ofelectrostatic attraction between adjacent interdigitating fingers. Thatis, motion of the truss comb structure 306 can be generated bydeveloping a voltage difference between the drive comb structure 302 andthe truss comb structure 306. In the case of drive portion 300, thevoltages applied at comb structures 302, 306 are also seen at fingers308, 310, respectively. The resulting voltage difference generates anattractive force between fingers 308 and 310. If the generatedelectrostatic force between fingers 308 and 310 is sufficiently large toovercome the other forces operating on truss comb structure 306 (such asa spring constant of resilient component 312), the electrostatic forcewill cause the motion of the truss comb structure 306 between a firstinterdigitated position (resting position at a zero voltage difference)and a second interdigitated position (position at a non-zero voltagedifference) among motion axis 320. Once the voltage difference isreduced to zero, resilient component 312 restores the position of trusscomb structure 306 to the first interdigitating position.

As shown in FIG. 3, each finger 310 in truss comb structure 306 can bedisposed between two fingers 308 of drive comb structure 302.Accordingly, an electrostatic force is generated on both sides of finger310 when a voltage difference is developed between comb structures 302and 306. Therefore, to ensure movement of truss comb structure 306 inonly one direction in response to a voltage difference, fingers 310 arepositioned with respect to fingers 308 such that the electrostatic forcein a first direction along the X-axis is greater than the electrostaticforce in an opposite direction in the X-axis. This is accomplished byconfiguring the finger spacing (i.e., spacing between fingers ofinterdigitated comb structures) in the first direction along the X-axis(x₀) and the finger spacing in the opposite direction along the X-axis(y₀) to be different when the voltage difference is zero. Since theamount of electrostatic force is inversely proportional to the distancebetween fingers, the motion of truss comb structure will be in thedirection associated with the smaller of x₀ and y₀.

The drive portion 300 provides a control mechanism for horizontalactuation in a shunt varactor that can be precisely controlled byadjusting the voltage difference between the drive and truss combstructures. This allows continuous adjustment over a range ofinterdigitating positions (by adjusting the voltage continuously over avoltage range).

Although the drive portion described above could be coupled to anyvariety of devices, using such a drive portion for various types ofdevices will only provide a partial improvement in manufacturingrobustness and device reliability. In general, the robustness of the ICfabrication techniques used for fabricating MEMS devices and other typesof devices is increased by reducing the variety of feature types anddimensional variation in each layer. The present invention exploits thischaracteristic. In particular, another aspect of the invention is to usethe comb structure drive portion in conjunction with a comb structurebased varactor portion, as shown below in FIG. 4.

FIG. 4 shows a top-down view of an exemplary MEMS shunt varactor 400that is useful for understanding the present invention. Each of theshunt varactors 102, 104, 106 of FIG. 1 can be the same as or similar tothe MEMS shunt varactor 400 of FIG. 4. As shown in FIG. 4, varactor 400includes a drive portion 401, similar to the drive portion 300 describedabove in relation to FIG. 3. That is, drive portion 401 includes drivecomb structures 402 a and 402 b (collectively 402), a truss combstructure 404, drive fingers 406, and truss fingers 408.

Truss comb structure 404 also includes resilient portions 410 with fixedends 412 a and 412 b (collectively 412). Resilient portions 410 compriseresilient or flexible reed structures 411 mechanically coupling trusscomb structure 404 to fixed ends 412. Therefore, a leaf spring structureis effectively formed on the two ends of truss comb structure. Inoperation, as a force is exerted on truss comb structure 404 (bygenerating a voltage difference between fingers 406 and 408) the reedstructures 411 deform to allow truss comb structure to move along motionaxis 405 from a first interdigitated position to at least a secondinterdigitated position. Once the force is no longer being exerted, thereed structures 411 apply a restorative force to restore the position ofthe truss comb structure 404 to a first interdigitated position. Theoperation and configuration of components 402-410 is substantiallysimilar to that of components 302, 306, 308, 310, 312 of FIG. 3.Therefore, the discussion of FIG. 3 is sufficient for describing theoperation and configuration for components 402-410 of FIG. 4. Asdescribed above, in addition to the drive portion 401, varactor 400 alsoincludes a variable capacitor or varactor portion 414, as shown in FIG.4. The varactor portion 414 includes input/output comb structures 416 aand 416 b (collectively 416) having a fixed position. The input/outputcomb structures 416 can also have one or more sense fingers 418extending therefrom. Within the varactor portion 414 of varactor 400,the truss comb structure 404 can additionally include one or moreadditional truss fingers 420 extending therefrom and interdigitatingsense fingers 418. Therefore, the truss comb structure 404interdigitates (via fingers 408 and fingers 420) both the drive fingers406 and the sense fingers 418. As a result, the truss comb structure 404mechanically connects and is part of both the drive portion 401 and thevaractor portion 414.

Fingers 406, 408, 418 and 420 are shown to be similarly dimensioned andhaving a same amount of overlap. However, the invention is not limitedin this regard and dimensional relationships can be different in thedrive portion 401 and varactor portion 414. Furthermore, the dimensionalrelationship can also vary within the varactor portion 414.Additionally, as described above with respect to FIG. 3, the combstructures 402, 404 and 416 can further include conductive portions andstructural portions, comprising non-conductive or semi-conductivematerials, to provide structure support for the conductive portions.

As described above, varactor 400 is configured to provide functionalityas a variable capacitor or varactor. In particular, the truss combstructure 404 is configured to provide an adjustable capacitance basedon adjustment of the gap between the first capacitor plate, provided byfingers 418, and a second capacitor plate, provided by fingers 420.Therefore, varactor 400 forms a first adjustable capacitor or varactorbetween truss comb structure 416 a and truss comb structure 404, with acapacitance of C_(OUT1), and a second adjustable capacitor or varactorbetween comb structure 416 b and truss comb structure 404, with acapacitance of C_(OUT2).

These first and second varactors can be used separately or incombination. In combination, these varactors can be connected to providecapacitance in series or parallel. For example, to provide a seriescapacitance, the capacitance can be measured between comb structures 416a and 416 b. In contrast to provide a parallel capacitance, thecapacitance can be measured between comb structures 416 a, 416 b andfixed end 412 a (if electrically coupled to fingers 420).

In some scenarios, a discontinuity 424 is provided to isolate fingers420 from fingers 408. As described above, the discontinuity 424 can beprovided to reduce any interference between the varactor portion 414 andthe drive portion 401. For example, to prevent the charge stored betweenfingers 418 and 420 from affecting a voltage difference between fingers406 and 408 and vice versa. However, if fixed ends 412 a and 412 b areboth coupled to ground, isolation between drive portion 401 and varactorportion 414 is maintained without requiring such discontinuity 424.

Varactor 400 operates as follows. A circuit (not shown) is connected tocomb structures 416 a, 416 b, and fixed end 412 a (if necessary, asdescribed above). To increase amount of capacitance at C_(OUT1) andC_(OUT2), a voltage difference (V_(BIAS)) is developed between fingers406 and 408 to generate electrostatic attraction between these fingers.For example, V_(BIAS) is applied across drive comb structures 402 andfixed ends 412 b (which is electrically coupled to fingers 408) to causesufficient electrostatic attraction between fingers 406 and 408 toinduce motion of truss comb structure 404, and consequently motion offingers 420 towards fingers 418, reducing a spacing X₀ _(_) _(CAP)between fingers 418 and 420. Consequently, the changing of the spacingbetween the capacitor plates results in a different capacitance valuefor both C_(OUT1) and C_(OUT2). Therefore, to increase capacitance,V_(BIAS) is selected to create an electrostatic force that is at leastgreater than the restorative force of reed structures 411 to causemotion of truss comb structure 404 along motion axis 405. Afterwards, todecrease the capacitance, V_(BIAS) is reduces such that theelectrostatic force is less than the restoring force applied by reedstructures 411. The restoring force then acts on truss comb structure404 to increase the gap between fingers 420 and fingers 418, and thuslower the capacitance.

The RF filter structure described above can be fabricated using a MEMSfabrication technique. This is illustrated in FIGS. 5A-5Z. FIGS. 5A-5Zshow partial cross-sections of a MEMS device (e.g., the MEMS deviceshown in FIG. 2) during various steps of a fabrication process inaccordance with embodiments of the present invention.

Manufacture of the MEMS device begins with the formation of an interfacelayer 502 on a substrate 500. An isolation layer 504 may also exist onthe substrate 500. After the formation of the interface layer 502,various steps are performed to fabricate an RF filter that is suspendedthere above. These steps are described below in relation to FIGS. 5B-5Z.

As shown in FIG. 5B, a first and second resist layers 508 a and 508 b(collectively 508) are disposed on the top surface of the substrate 500so as to cover the circuitry 506. Next, the second resist layer 508 b ispatterned to form at least partially the shunt varactors (e.g., a shuntvaractor 102, 104 and/or 106 of FIG. 1), inductors (e.g., inductor 108,110, 112, 114 and/or 116 of FIG. 1), and a transition portion (e.g.,transition portion 118 of FIG. 1) of the RF filter (e.g., RF filter 100of FIG. 1). A schematic illustration of second resist layer 508 b whichhas been patterned is provided in FIG. 5C. As shown in FIG. 5C, at leastthree patterns have been formed in the second resist layer 508 b. Afirst pattern 510 is provided for forming a lower portion of a combstructure of a shunt varactor. Pattern 512 is provided for forming alower portion of an inductor coil. Pattern 514 is provided for forming alower portion of a transition portion. Therefore, each pattern 510, 512,514 is then filled with a conductive material 516-520, as shown in FIG.5D.

In FIG. 5E, a third resist layer 522 is disposed over the first andsecond resist layers 508 and conductive material 516-520. The thirdresist layer 522 is then patterned in FIG. 5F for forming at least aportion of a middle section of the comb structure, inductor coil, andtransition portion. As such, three patterns 524-528 are formed in thethird resist layer 522. Pattern 524 is provided for forming a portion ofmiddle section of the comb structure of the shunt varactor. Pattern 526is provided for forming a portion of a middle section of an inductorcoil. Pattern 528 is provided for forming a portion of a middle sectionof a transition portion. Therefore, each pattern 530, 532, 534 is thenfilled with the conductive material 536-540, as shown in FIG. 5G.

In FIG. 5H, a fourth resist layer 537 is disposed over the third resistlayer 522 and conductive material 536-540. The fourth resist layer 537is then patterned in FIG. 5I for forming a dielectric strap which willsupport a center conductor (e.g., center conductor 122 of FIG. 1) withinthe ground structure (e.g., ground structure 120 of FIG. 1) of thetransition portion. Therefore, pattern 546 is then filled with anon-conductive material 552 as shown in FIG. 5J.

This process of disposing, patterning and filling of resists layers isrepeated as shown in FIGS. 5K-5Y until the RF filter structure of FIG.5Y is formed. Subsequently, the resist layers are removed as shown inFIG. 5Z. As a result of removing the resists layers, the RF filterstructure is suspended over the substrate 500. The RF filter iselectrically isolated from the circuitry 502 via air. A schematicillustration of an exemplary RF filter suspended over a substrate isshown in FIG. 2 which was discussed above.

Although the invention has been illustrated and described with respectto one or more implementations, equivalent alterations and modificationswill occur to others skilled in the art upon the reading andunderstanding of this specification and the annexed drawings. Inaddition, while a particular feature of the invention may have beendisclosed with respect to only one of several implementations, suchfeature may be combined with one or more other features of the otherimplementations as may be desired and advantageous for any given orparticular application. Thus, the breadth and scope of the presentinvention should not be limited by any of the above describedembodiments. Rather, the scope of the invention should be defined inaccordance with the following claims and their equivalents.

I claim:
 1. A method for making an integrated MicroelectromechanicalSystems (“MEMS”) device, comprising: forming first electronic circuitryon a major surface of a substrate; forming a MEMS filter structure onthe major surface of the substrate; removing at least one first resistlayer from the MEMS filter structure to form (a) a MEMS filter devicesuspended over the major surface of the substrate exclusively by atransition portion electrically connecting the MEMS filter device to thefirst electronic circuitry, and (b) a gas gap between the MEMS filterdevice and the major surface of the substrate; and removing at least onesecond resist layer from the MEMS filter structure such that thetransition portion is defined by a three dimensional hollow groundstructure in which an elongate center conductor is suspended andenclosed except for at one or more points of connection between thecenter conductor and the MEMS filter device; wherein at least a portionof the MEMS filter structure resides external to the three dimensionalhollow ground structure of the transition portion, and the threedimensional hollow ground structure comprises at least one elongatelinear portion extending between the first electronic circuitry and theMEMS filter.
 2. The method according to claim 1, further comprisingforming second electronic circuitry on the substrate so as to residebetween the substrate and the MEMS filter device, wherein a gas gapexists between the second electronic circuitry and the MEMS filterdevice.
 3. The method according to claim 2, wherein the secondelectronic circuitry is formed using a first conductive material havinga first melting point, and the MEMS filter device is formed using asecond conductive material having a second melting point that isdifferent from the first melting point by no more than 100° C.
 4. Themethod according to claim 1, further comprising connecting a dielectricstrap between two opposing sidewalls of the transition portion wherebythe elongate center conductor can be suspended within the threedimensional hollow ground structure.
 5. The method according to claim 1,further comprising providing a gas gap between the elongate centerconductor and the three dimensional hollow ground structure.
 6. Themethod according to claim 1, further comprising electrically isolatingat least two adjacent electronic elements of an RF MEMS filter devicefrom each other using two spaced apart sidewalls of the ground structureof the transition portion.
 7. The method according to claim 1, furthercomprising electrically isolating at least two adjacent electronicelements of an RF MEMS filter device from each other using a commonsidewall of the ground structure of the transition portion.
 8. A methodfor making an integrated Microelectromechanical Systems (“MEMS”) device,comprising: forming first electronic circuitry on a major surface of asubstrate; forming a MEMS filter structure on the major surface of thesubstrate; removing at least one first resist layer from the MEMS filterstructure to form (a) a MEMS filter device suspended over the majorsurface of the substrate exclusively by a transition portionelectrically connecting the MEMS filter device to the first electroniccircuitry, and (b) a gas gap between the MEMS filter device and themajor surface of the substrate; removing at least one second resistlayer from the MEMS filter structure such that the transition portion isdefined by a three dimensional hollow ground structure in which anelongate center conductor is suspended; and electrically isolating atleast two adjacent electronic elements of an RF MEMS filter device fromeach other using a common sidewall of the ground structure of thetransition portion; wherein the common sidewall has a thickness that isgreater than at least one other second adjoining sidewall of the groundstructure.